Hardening accelerators are admixtures that are widely used in construction to increase the mechanical strength of cementitious mixes at early curing times. The need for their use arises from the demand of a quicker formwork turnover or from the requirement of reducing construction times when the concrete elements are subject to very heavy loads within a few days of placement. The use of hardening accelerators is most common in cold geographical areas or seasons. Low temperatures slow down cement hydration and this implies longer curing times to gain the compressive strength necessary for self-standing structures. Hardening accelerators boost compressive strength at early curing time because they speed up cement hydration without affecting the total amount of cement that reacts with water at longer hydration times, thus leaving unchanged the final compressive strength of the material and its durability. It is known that many inorganic compounds, such as chlorides, fluorides, carbonates, nitrates, nitrites, thiosulfates, and thiocyanates have accelerating properties. Organic accelerating compounds include triethanolamine, diethanolamine, urea, glyoxal, and formates. Chlorides and nitrates, more specifically calcium chloride and calcium nitrate, are among the most effective accelerators; unfortunately, these compounds favor corrosion of rebar and their use is therefore precluded for reinforced concrete. Calcium formate is not harmful to rebar and it acts as a hardening accelerator when it is added in a small percentage with respect to the cement weight. The main drawback of calcium formate, though, is its low solubility in water, which makes it difficult to use it as a solution; it is then necessary to add it as a powder with some practical difficulties arising from the typical fineness of its dust. Organic molecules, such as triethanolamine, act as accelerators for cement hydration at low doses (0.025% with respect to cement), while they show retarding properties at higher doses (0.06% with respect to cement). The mechanism of action of accelerators is not yet clear, but it is assumed that they accelerate the hydration of C3S through surface adsorption, ion chelation, precipitation of insoluble salts and modification of the microstructure of hydrated phases. A wide review of scientific and patent literature on the subject can be found in Collepardi, M., “Scienza e Tecnologia del Calcestruzzo”, Hoepli Ed., Milano, 1987, 335-337, in Ramachandran, V. S., “Concrete Admixtures Handbook—Second edition”, Noyes Publications, Park Ridge, N.Y., 1995, 185-273 and 1047-1049 and Cheung, J. et al., “Impact of Admixtures on the Hydration Kinetics of Portland Cement”, Cement and Concrete Research, 41, 2011, 1289-1309. The accelerating effect is mainly exerted on the silicon-containing phases of cement and, in particular, on tricalcium silicate, 3CaO.SiO2, the most important component of portland cement. The importance of tricalcium silicate, indicated in cement chemistry as C3S (C═CaO, S═SiO2), is due both to the fact that it is the most abundant component of portland cement (50-70%) and to its contribution to hardening of the cement paste. It is in fact thanks to the reaction of C3S with water that cement hardens and changes, within a few hours, from a plastic pourable mass to a solid conglomerate that can withstand great mechanical strain. The hydration reaction of tricalcium silicate can be written as:C3S+(3−x+y).H2O→(3−x).Ca(OH)2+CxSHy 
The product formed by hydration of tricalcium silicate, calcium silicate hydrate, is an amorphous compound of undetermined composition, in which the proportion between the different constituents (C═CaO, S═SiO2, H═H2O) varies depending on curing time and conditions. For this reason and because of the lack of a well-defined crystal structure, this compound is generally indicated as “calcium silicate hydrate gel,” with the notation C—S—H. C—S—H is a porous product that covers the cement grains and is characterized by a great surface: it is present as a bundle of fibrous particles of a few microns in length and some tenths of micron of thickness, whose intertwining contributes to determining the binding properties of cement. It is reported in the literature that C—S—H itself can act as a hardening accelerator for C3S (Kondo, R., Daimon, M., J. Am. Ceram. Soc. 52, 1969). The findings of Kondo et al. have recently been confirmed using synthetic C—S—H (Thomas, J. J. et al., J. Phys. Chem., 113, 2009, 4327-4334). It was demonstrated that adding C—S—H to a portland cement paste serves as a crystallization source for the C—S—H produced during the hydration of C3S, thus accelerating nucleation and precipitation both on cement grains and in the pore solution. This phenomenon has a beneficial effect on the development of early mechanical strength and on durability of hardened conglomerates. WO2010026155 describes a process for the production of C—S—H used as a hardening accelerator, in which precipitation of calcium silicate hydrate from aqueous solutions containing calcium and silicate ions is obtained in the presence of a superplasticizing grafted polymer (comb polymer) for hydraulic binders. WO2011026720 describes a process for the production of C—S—H used as a hardening accelerator, in which precipitation of calcium silicate hydrate from aqueous solutions containing calcium and silicate ions is obtained in the presence of a polycondensated polymer containing both aromatic or heteroaromatic structural units bearing polyether side chains and aromatic or heteroaromatic units bearing phosphate groups. WO2011026723 describes a process for the production of C—S—H used as a hardening accelerator, in which precipitation of calcium silicate hydrate from aqueous solutions containing calcium and silicate ions is obtained in the presence of an aqueous solution of a water-soluble polymer containing sulfonic and aromatic groups. WO201104347 describes a process for the production of C—S—H used as a hardening accelerator, in which precipitation of calcium silicate hydrate from aqueous solutions containing calcium and silicate ions is obtained in the presence of a water-soluble dispersant containing at least one polyoxyethylene structural unit with a terminal functional group capable of acting as an anchor group on the surface of cement particles.